N-Ethylmaleimide inactivates maleylacetone cis-tran isomerase. Experiments designed to find the site of attachment will be carried out. Possible proton exchange at the isomerizing carbon-carbon double bond of maleylacetone will be looked for during enzymic catalysis. If exchange is absent deuterated substrate will be synthesized and secondary alpha deuterium kinetic isotope effects determined to gain further insight about the mechanism. Further studies with inhibitors for the substrate and coenzyme are planned.